Due to great potential application to flexible display devices etc., the organic light emitting diode (OLED) device is attached great importance by the scientific community and display industry in recent years, and it is one of the hot spots of current research and development.
However, the OLED device technology is currently experiencing a bottleneck in the development process; a main issue is that light emitting efficiency and lifetime of the OLED device cannot meet practical demand so as to greatly limit the development of the OLED technology. Factors of affecting the luminous efficiency and service life of the OLED device are multifaceted, but the fluorescence efficiency and stability of the light emitting material directly affect the performance of the OLED device. A first generation light emitting material that converts electrical energy into light based on singlet state exciton only has 25% theoretical conversion efficiency, and the conversion efficiency thereof cannot be further increased. A second generation light emitting material (i.e. phosphorescent material) that converts electrical energy into light based on triplet state exciton has theoretical conversion efficiency up to 100%; however, the phosphorescent material has some drawbacks, such as blue phosphorescence of poor efficiency and short lifetime, unable to be resolved.
In 2012, professor Adachi at Kyushu University in Japan found a new material of thermally activated delayed fluorescence (TADF) based on triplet-singlet state transition, and TADF has conversion efficiency close to 100% and blue phosphorescence without the drawbacks as the phosphorescent material. Such new materials as TADF are known as third generation efficient light emitting material of OLED. In comparison with the phosphorescent material, the new material can provide high luminous efficiency without using the rare metals of high cost. Therefore, the new material has features of high luminous efficiency and low fabrication cost. It is estimated that fabrication cost of an OLED device using the new material is less than one tenth cost of an OLED using the conventional material such as the phosphorescent material.
The new material has just been discovered and immediately caused great concern to scientific community and display industry related thereto. However, the new material is very scarce, and the theory thereof is still imperfect, so the new material needs further research and development by researchers. Additionally, based on report, TADF has relatively low glass transition temperature and poor thermal stability.